This invention relates to the economical purification of water containing soluble and sparingly or partially soluble inorganic compounds using a single-stage membrane process with a unique recycle of ion exchange xe2x80x9csoftenedxe2x80x9d membrane concentrate stream.
Hardness compounds such as barium, calcium, magnesium, iron, silica, carbonate and bi-carbonate, fluoride and sulfate are commonly found in surface water supplies such as lakes and rivers as well as groundwater supplies such as water wells and aquifers and in aqueous industrial effluents and landfill leachates. Water containing hardness compounds is frequently purified by using water softeners and demineralizers in the form of xe2x80x9cion exchange resins, IXxe2x80x9d, chemical softeners using the cold lime or hot lime softening process, reverse osmosis (RO) membranes, nanofiltration (NF) membranes and/or distillation. Industry needs purified water containing low to very low concentrations of hardness compounds and of soluble inorganic compounds in order to supply their cooling towers, low-pressure and high pressure boilers, heat exchangers and various process uses. On the other hand, the pharmaceutical and electronics industries as well as hospitals and laboratories require high purity water that is almost completely free from inorganic compounds.
The conventional water treatment processes listed above are not suitable because, in the case of DX, the process involves the inefficient transfer of soluble and xe2x80x9csparingly solublexe2x80x9d water impurities to a resin bed which must be regenerated and/or disposed of at high cost. In the case of lime softening, large quantities of chemicals are added and large chemical waste volumes are generated. If conventional RO or NF membranes are used, substantial volumes of RO or NF membrane concentrates will be generated because the permeate recovery (percentage) from these processes is normally limited to approximately 70%-75% and the concentrates must therefore be treated further or disposed of at a large cost. Finally, the very high capital and/or operating costs associated with the direct application of distillation processes normally precludes the use of distillation as a single-step treatment method.
Although membrane filtration processes such as reverse osmosis (RO) and nanofiltration (NF) provide effective and economically viable methods for purifying water, these membrane processes are currently limited in the percentage of purified water produced (known as the permeate recovery or product recovery), by scale formation. Most of the soluble and scale-forming compounds are separated by the membranes and concentrated into a smaller volume, typically 20-30% of the volume of the original water stream. This membrane concentrate volume is normally too large and too costly to dispose of, except in the case of seawater desalination applications where the concentrate stream is returned to sea or where there are no regulatory limits on the concentration of inorganic compounds in the effluent. The main reason why further recovery of purified water from xe2x80x9cconventionalxe2x80x9d RO and NF membranes is not possible is the tendency of inorganic scale such as calcium carbonate and silica to form on the surface of the membranes as the concentration of these compounds is increased beyond their saturation values. Deposition of such compounds frequently results in the loss of purified water production (also known as loss of permeate flux through the membrane) and the eventual need for costly replacement of the membranes.
The use of chemical additives in the water supply such as acids to reduce the pH and inorganic or organic scale inhibitor compounds is practiced in the water treatment arid membrane industry in order to provide some improvement in the water recovery and prevent scale formation. However, such improvement is of limited extent since no scale inhibitor is effective for all the contaminants nor for all permeate recovery ranges and therefore they do not represent an economically viable option for the treatment of the entire water stream.
A survey of prior art shows the following patents:
U.S. Pat. No. 4,000,065 discloses the use of a combination of reverse osmosis (RO) and ultrafiltration (UF) to separate organic material from the aqueous stream. The contaminated aqueous stream is circulated from the high pressure compartment of an RO unit to the high pressure compartment of a UF unit, then to the low pressure compartment of the UF unit and then back to the high pressure compartment of the RO unit.
Japanese Patent 57-197085 discloses a filtration apparatus that comprises connecting UF apparatus and RO apparatus in series so as not to deposit scale on the RO membrane.
U.S. Pat. No. 3,799,806 discloses purification of sugar juices by repeated ultrafiltration and reverse osmosis purification steps.
U.S. Pat. No. 4,083, 779 discloses a process for treatment of anthocyanate extract by ultrafiltration and reverse osmosis treatments.
U.S. Pat. No. 4,775,477 discloses a process for extraction of cranberry presscake wherein the presscake is ground and subjected to microfiltration to remove colloidal high molecular weight compounds followed by reverse osmosis to recover a red-colored solution.
U.S. Pat. No. 5,182,023 discloses a process for removing arsenic from water wherein the water is first filtered to remove solids then passed through an ultrafilter, followed by a chemical treatment to adjust pH to a range from about 6 to 8. Thereafter, scale-inhibitors and anti-fouling materials are added before subjecting the water to reverse osmosis to provide a stream having less than about 50 ppb of arsenic.
Japanese Patent 53025-280 discloses the separation of inorganic and organic compounds from a liquid by first using a reverse osmosis membrane and then using a second reverse osmosis membrane having a more permeable membrane such as a microporous or ultrafiltration membrane. Part of the contaminated liquid obtained from the first membrane is processed through the second membrane.
U.S. Pat. No. 5,501,798 discloses a high recovery water purification process involving the use of reverse osmosis followed by chemical precipitation of hardness compounds from the RO concentrate followed by microfiltration to separate precipitated solids and recycling of the xe2x80x9csuspended solids"" free concentratexe2x80x9d back to the RO.
The above-referenced patents and other available literature have aimed at preventing precipitation of inorganic scale and other membrane fouling compounds as the water is treated by reverse osmosis membranes. As the concentration of scale and other inorganic and organic fouling compounds build up on the surface of the RO membranes, the purified water permeation rate deteriorates with eventual irreversible loss of productivity and need for costly membrane replacement. Prior art teaches acidification (i.e. pH reduction) as a means of reducing the potential of calcium carbonate scale formation. Prior art also teaches the addition of scale inhibitors such as polyacrylic acids and sequestering agents such as ethylene diamine tetracetic acid (EDTA) and sodium hexametaphosphate (SHMP) in order to reduce the scale formation potential due to barium sulfate, calcium fluoride, calcium and magnesium carbonate and sulfate and silica. However, these scale inhibitor compounds are not sufficiently efficient to allow very high water recoveries and concentration factors to be achieved. Maximum recoveries in the presence of scale inhibitors may be in the range 70%-75%, based on the treatment of hard xe2x80x9cwell-waterxe2x80x9d. The above-referenced patents also teach the separation of suspended solids existing originally in natural water sources and industrial effluents or the separation of chemically-precipitated compounds using ultrafiltration or microfiltration before reverse osmosis treatment. While removal of suspended solids by membrane filtration will prevent fouling of the RO membranes, it does not prevent concentration and eventual deposition of the initially soluble scale compounds, as the recovery of purified water is increased using RO. U.S. Pat. No. 5,501,798 teaches a high recovery process involving the use of a single stage reverse osmosis system, chemical precipitation and microfiltration (MF) and recycling of MF permeate to the RO membrane system to maximize the recovery of purified water, However, high water recovery from the process of U.S. Pat. No. 5,501,798 is limited by the maximum RO membrane system operating pressure of 1,000 psig. The process of U.S. Pat. No. 5,501,798 also has the inherent disadvantage of high capital and operating cost associated with the chemical precipitation step and the suspended solids separation step. The suspended solids in the supernatant from the precipitation step are separated by microfiltration membranes or other separation means and the softened MF membrane permeate or filtrate is recycled to the RO membranes in order to minimize the scale formation potential on these membrane.
Therefore, there is need for an improved, simple, economical, single-stage process for the purification of small flowrates of water (i.e. 5-50 U.S. gallons/minute., gpm), which provides high water recoveries, in the range 67%-99.9%, while preventing formation of scale on the RO or nanofiltration (NF) membrane surfaces, and thus prolonging the useful life of these membranes.
It is an object of the invention to provide an improved process for the treatment of small flowrates of water from natural sources or wastewater containing soluble and partially or xe2x80x9csparingly solublexe2x80x9d inorganic scale-forming compounds.
It is another object of the invention to provide an improved process using a simple, single stage membrane system to purify water containing soluble and sparingly soluble inorganic compounds.
It is still another object of the invention to provide an improved process using a single-stage membrane system to purify water containing soluble and sparingly soluble inorganic compounds wherein 67%-99.9% of the water can be recovered as purified water without incurring the risk of scale deposition which reduces the useful life of the membrane.
Yet, it is another object of the invention to provide an improved process using a single-stage membrane system to purify water containing soluble and sparingly soluble inorganic compounds wherein 67%-99.9% of the water can be recovered as purified water without incurring the risk of scale deposition and wherein the concentrate (also known as retentate) is softened and recycled for further processing using a suitable, low-cost ion exchange water softening resin to remove the cationic scale precursors.
These and other objects of this invention will become clear from the specification, claims and drawings appended hereto.
In accordance with these objects, there is provided an improved, simple, single stage economical membrane process for the treatment of small flowrates of water from natural sources or wastewater containing partially or xe2x80x9csparingly solublexe2x80x9d inorganic scale-forming compounds wherein very high permeate recoveries in the range 67%-99.9% are achieved. The water containing the inorganic scale-forming compounds as well as completely soluble inorganic compounds is first pre-treated in order to separate virtually all the suspended matter including oil and grease by using an oil separation device followed by chemical coagulation, flocculation, clarification and/or gravity settling, and multi-media filtration. Alternatively, the water is pre-filtered using micro-filtration or ultra-filtration to effectively separate all suspended solids and colloidal matter. If iron is also present in the water, the water must be aerated using appropriate in-tank air spargers before coagulation, flocculation and/or filtration. The pre-treated water in the present invention is purified by the single stage membrane system operating at a suitable pressure depending on the concentration of soluble compounds and the desired permeate recovery. In order to prevent formation of scale compounds on the membranes, the concentrate (also known as retentate) is softened using a suitable, low-cost ion exchange (IX) water softening resin to remove the cationic scale precursors. The softened concentrate will contain a high concentration of soluble ions, also known as the total dissolved solids (TDS). The softened membrane concentrate is recycled and blended with the influent water stream to undergo further purification. A small volume of concentrate is removed from the system upstream from the IX water softening resin and disposed of as a reject stream in order to control the membrane""s osmotic pressure and the permeate recovery. By using an efficient IX water softening resin in lieu of chemical precipitation of hardness compounds, formation of precipitated solids which is encountered in the prior art is avoided, thereby simplifying the post-treatment of the membrane concentrate and reducing the overall complexity as well as the capital and operating costs of the process.